1. Field of the Invention
The present invention relates to the field of regulated electronic systems, and more specifically to integrated circuits including an error amplifier as an input of a regulation system.
2. Discussion of the Related Art
An example of application of the present invention is the regulation of a supply voltage, for example, in a voltage converter of switched-mode type power supply. In such a converter, an error amplifier is used at the input of a circuit for controlling a pulse-width modulated switch. This amplifier compares a voltage proportional to the voltage provided by the converter to a reference voltage, and provides an analog signal proportional to the difference between the two voltages. More generally, the present invention applies to any electronic regulation using a differential amplifier of an electric signal representing an image of the signal regulated with respect to an internal reference.
FIG. 1 shows a conventional example of an integrated circuit error amplifier. Such an amplifier uses an operational amplifier 1, an output of which provides an error signal e. As an input, amplifier 1 receives a reference voltage Vref which is compared to a voltage applied to its other input. Voltage Vref is provided by a voltage reference circuit 2 (REF), the output of which is connected to the non-inverting input of amplifier 1. Circuit 2 is generally integrated with amplifier 1 in the same circuit. In FIG. 1, the limit of the integrated circuit has been schematically illustrated by dotted lines 3. Other components are of course integrated with amplifier 1. However, the amplifier requires two terminals 4 and 5 of external connection to integrated circuit 3. Terminal 5 represents the output of amplifier 1. Terminal 4 represents the non-inverting input of amplifier 1. A voltage Vreg, which is a function of the regulated variable, is applied to terminals 6 and 7 of a dividing bridge formed of a resistor R1 and of a resistor R2 in series, terminal 7 representing the ground. Midpoint 8 of this series connection is connected to terminal 4. The resistive dividing bridge is necessary since the voltage reference of amplifier 1 is integrated. The adaptation of the proportionality coefficient between reference voltage Vref and voltage Vreg to be regulated is performed by the choice of resistances R1 and R2. The relation linking the regulated voltage to the reference voltage is: Vreg=(1+R1/R2) Vref.
The feedback loop of amplifier 1 is formed of a resistor R3 in series with a capacitor C3. Resistor R3 and capacitor C3 of the loop are generally external to circuit 1.
An error amplifier such as illustrated in FIG. 1 has a transfer function of proportional-integral type. The proportionality coefficient of the transfer function is given by the ratio between resistances R1 and R3. The integration constant is given by the product of capacitance C3 by resistance R1.
An error amplifier such as illustrated in FIG. 1 is present, for example, in an integrated circuit sold by STMicroelectronics Company under trade name UC3842. The output (not shown) of this integrated circuit provides a pulse train to a switch to form a switched-mode converter. Voltage Vreg then corresponds to the voltage provided by the converter.
A disadvantage of conventional error amplifiers is that the forming of an integrated reference voltage source 2 requires specific settings and adjustments upon manufacturing of the integrated circuit. This adjustment of reference voltage Vref is most often performed by a resistive fuse network. The reference voltage source is generally obtained from a reference current source further used in the integrated circuit to bias the other stages. This current source is itself adjusted, for example, by a resistive fuse network. The use of two reference magnitudes makes the circuit complex and increases the test cost of the circuit.
Another disadvantage of the error amplifier of FIG. 1 is that resistances R3 and C3 cannot be integrated. This is due to the high values of these components, for reasons of stability of the regulation loop in which amplifier 1 is placed. Capacitor C3 has a value (typically, several tens of nanofarads) much greater than a few tens of picofarads, which is generally considered as an integration limit.
Another disadvantage is that two terminals 4 and 5 of access to integrated circuit 3 are necessary to implement the error amplifier.
The present invention aims at overcoming at least one of the disadvantages of a known error amplifier.
The present invention more specifically aims at providing a novel error amplifier which no longer needs a reference voltage.
The present invention also aims at providing a solution which reduces or minimizes the number of external components of a circuit integrating the error amplifier.
The present invention further aims at reducing or minimizing the number of necessary terminals of the integrated circuit.
To achieve these and other objects, the present invention provides an error amplifier providing an analog error signal, including an operational amplifier, an output terminal of which controls an active load of discharge of a resistive and capacitive network supplied by a source of a reference current, and means for copying the reference current in an input resistor, a terminal of which receives a measurement signal and the other terminal of which is connected to a non-inverting input of the operational amplifier, the error signal being available across the resistive and capacitive network.
According to an embodiment of the present invention, the transfer function is set by the values of the resistive and capacitive network.
According to an embodiment of the present invention, the inverting and non-inverting terminals of the operational amplifier are each connected, by a resistor, to a reference potential.
According to an embodiment of the present invention, a first power terminal of the active load is connected, by the reference current source, to the supply voltage of the integrated circuit and forms an output terminal providing the error signal.
According to an embodiment of the present invention, the inverting input of the operational amplifier is connected to a second power terminal of the active load.
According to an embodiment of the present invention, the network is formed of a resistor in parallel with a capacitor, connected between the first power terminal of the active load and the reference potential.
According to an embodiment of the present invention, the error amplifier is made in the form of an integrated circuit, all resistors and capacitors forming it being integrated except for the input resistor.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.